Reverse osmosis systems particularly serve to recover pure sterile water from tap water, e.g. for medical, pharmaceutical and food-technological applications.
As is known, its functional principle consists in that the water to be treated is passed in a filter module under pressure along the surface of a semipermeable membrane, wherein part of the water, the so-called permeate, passes through the membrane and is collected at the other side of the membrane as ultrapure water and supplied to the point of consumption.
The part of untreated water that does not pass through the membrane and is enriched with retained substances, the so-called concentrate, flows out of the membrane module at the end of the flow path of the primary chamber.
Ideally, the ultrapure water obtained thereby is sterile due to the retention characteristics of the membrane and free of organic decomposition products. In reality, however, this is just not the case. Without special counter-measures there may be an infestation of the permeate system with microorganisms. A so-called biofilm is formed on the inner surfaces of the liquid conducting system. Said biofilm is also called fouling.
Fouling stands for the loss in permeate capacity due to deposition of secondary layers on the membrane surface. This may be organic material, colloidal substances, or inorganic salts that exceed the precipitation limits upon concentration.
Up to the present day there has been no generally applicable recipe for preventing fouling. “Low fouling membranes” and an improved pretreatment as well as better membrane cleaning techniques just represent inadequate technical possibilities of controlling fouling.
In industry, temperature-resistant polymer membranes for reverse osmoses are for instance available; these are disinfected with water at 90°. First of all, said measure only serves to reduce the number of germs, but is of little help for removing the biofilm.
Another possibility consists in carrying out disinfection or purification in reverse osmosis systems at suitable time intervals. To this end the normal operation is interrupted and a chemical disinfecting or cleaning agent is supplied to the liquid conducting system. Following an appropriate exposure time a flushing operation is performed, which serves to remove the introduced disinfecting or cleaning agent and its reaction products again, so that the normal supply operation can then be resumed again.
On account of the great risks associated with an uncontrolled supply and with the residues of disinfecting or cleaning agents, such operation normally calls for the employment of technical stuff, especially in the case of medical applications (hemodialysis), and is thus cost-intensive.
Further disadvantages of the former solutions are the high energy input during thermal disinfection and the loss in permeate output due to the unremovable biofilm.
Since the loss in permeate output or a contamination with germs can even not be compensated by complicated cleaning and disinfecting measures in many cases, the membranes are exchanged.
Such a membrane exchange is carried out in today's devices by a technician in such a way that first of all the whole reverse osmosis system is stopped and the tripartite filter module, made up of membrane element, pressure pipe and connection unit, is disassembled by means of a tool. A conventional construction is shown in FIG. 1.
Thereupon, the water-wetted membrane element is pulled out of the pressure pipe and replaced by a new one. Depending on the size of the system, several membrane elements are concerned.
Several liters of water, also contaminated one, may here exit per membrane element.
The standstill times of the reverse osmosis system caused during repair work may here also be considerably long and very disadvantageous for patients, particularly in the sector of organ-supporting devices (hemodialysis).
A further drawback is the chemical disinfection following the exchange, which is needed for the reason that during repair even the ultrapure components of the connection pipes or components were contaminated by the technical stuff and by tools.
Of considerable disadvantage is the tripartite structure of membrane, pressure pipe and connection unit of the existing filter module; this tripartite structure has a historical background and is due to the originally high transmembrane pressures of the membranes. Therefore, filter modules have been devised with a high pressure resistance of pressure pipe and connection unit.
95% of all RO membranes are nowadays crosslinked aromatic diamines (-polyimide-).
This aromatic polyamide is applied in an extremely thin layer (<0.3 micrometers) to a carrier membrane (or support layer). The membrane is therefore also called thin-film membrane.
Since the membrane layers are getting thinner and thinner at the same permeate output, this results in a transmembrane pressure that is getting smaller and smaller. The existing pressure pipe designs exploit this innovation from an economic viewpoint only inadequately.
Especially in the sector for medical and food-technological applications, attention is paid that there is no dead space in the filter module. To this end so-called “full fit” membranes without any dead space are offered by the industry. These membranes are expensive. An additional drawback is the necessary additional pump capacity that is needed for overflowing the pipe gap between the membrane element and the pressure pipe.
Another drawback of this technology is the fact that the liquid feeding and discharging connections on the filter module are made two-sided on both ends of the filter module.
Other drawbacks of the former solution are that the components for measuring state variables, throughputs, or substance characteristics are not centrally arranged between the pipelines. These components are e.g. conductivity or pH measuring cells and devices for changing the liquid streams, for instance valves or throttles.
The enormous piping and tubing efforts needed for stringing the filter modules so as to obtain a single-filter system must be evaluated in the same way.